Expandable microspheres and methods of making and using the same

ABSTRACT

Expandable microspheres formed by suspension polymerization using a shot growth method are provided. The microspheres are formed of a continuous, gas impermeable shell surrounding a blowing agent. The shell includes a first polymer layer formed from primary monomers and a second layer that includes a chemically reactive monomer or a high Tg monomer. To form the microspheres, the primary monomers are polymerized in a reaction vessel to an approximate 90% polymerization, at which time a secondary monomer that is either a monomer having a Tg of at least 85° C. or a chemically reactive monomer, is added to the reaction vessel to drive the polymerization reaction to completion. The outer layer thus contains either a larger amount of the high Tg monomer or a chemically reactive monomer that possesses the ability to covalently bond a cationic species. The microspheres may be used in papermaking processes to increase the paper bulk.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to and claims domestic priority benefitsfrom U.S. Provisional Patent Application Ser. No. 61/190,354 entitled“Expandable Microspheres And Methods Of Making And Using The Same” filedAug. 28, 2008, the entire content of which is expressly incorporatedherein by reference in its entirety.

TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION

The present invention relates generally to expandable microspheres foruse in papermaking processes, and more particularly, to thermallyexpandable microspheres formed by suspension polymerization utilizing ashot growth technique. A composition and paper substrate including theexpandable microspheres are also provided.

BACKGROUND OF THE INVENTION

The amount of costly cellulose fibers present in a paper substrate, inpart, determines the density of the substrate. Therefore, large amountsof these costly cellulose fibers present in a paper substrate produce amore dense substrate at high cost, while low amounts of cellulose fiberspresent in a paper substrate produce a less dense substrate at a lowcost. Reducing the density of a coated and/or uncoated paper product,board, and/or substrate inevitably leads to reduced production costs.This correlation remains true for all paper and substrate production anduses thereof, but is especially true, for example, in paper substratesused in envelopes, folding cartons, and in other packing applications.The substrates used in such envelope and packaging applications have aspecified thickness or caliper.

By reducing the density of the paper substrate at a target caliper, lesscellulose fibers are required to achieve the target caliper. In additionto a reduction in production costs, there is a production efficiencythat is appreciated and realized when a paper substrate's density isreduced. This production efficiency is due, at least in part, to areduction in drying requirements (e.g., time, labor, capital, etc.) ofthe paper substrate during production.

Examples of reducing density of the base paper substrate include the useof:

-   -   Multi-ply machines with bulky fibers, such as BCTMP and other        mechanical fibers in the center plies of paperboard;    -   Extended nip press section for reducing densification during        water removal; and    -   Alternative calendering technologies such as hot soft        calendering, hot steel calendering, steam moisturization, shoe        nip calendering, etc.        However, these potential solutions involve high capital and        costs. Thus, they may be economically infeasible.

Further, even if the above-mentioned costly reduction in density methodsare realized, thus producing a paper substrate having a target caliper,the substrate is only useful if such methodologies foster an acceptablysmooth and compressible surface of the paper substrate. Presently, thereare few potential low cost solutions to reduce the density of a papersubstrate that has an acceptable smoothness and compressibility so thatthe substrate has a significant reduction in print mottle and anacceptable smoothness.

Low density coated and uncoated paper products, board, and/or substratesare highly desirable from an aesthetic and economic perspective.Unfortunately, current methodologies produce substrates that have poorprint and/or printability quality. In addition, acceptable smoothnesstargets are difficult to attain using conventional methodologies.

One methodology to address the above-mentioned problems at lower cost isthe utilization of expandable microspheres in paper substrates. Thesemethodologies, in part, can be found in the following U.S. Pat. Nos.6,846,529; 6,802,938; 5,856,389; and 5,342,649 and in the following U.S.Patent Publications: 2008/0017338; 2007/0044929; 2007/0208093;2006/0000569; 2006/0102307; 2004/0065424; 2004/0052989; 2004/0249005 and2001/0038893. The contents of each of these patents and publications arehereby expressly incorporated by reference in their entirety.

Many microspheres are found, when applied to the papermaking process, tohave relatively low retention in the resultant paper substrate. As aresult, the expandable microspheres are lost to the white water in thepaper making process and the efficiency of the introduction of theexpandable microspheres into the resultant paper substrate is low. U.S.Patent Publication Serial No. 2007/0044929 attempts to increase theretention of the microspheres by creating a composition having a muchless negative charge than the base expandable microsphere compositionsknown previously.

Despite attempts to create a less dense and more bulky paper substrate,there remains a need in the art for a less costly and more efficientsolution to reduce density and increase bulk while maintaining goodperformance characteristics such as smoothness and print mottle in thepaper substrate.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an expandablemicrosphere that includes a gas impermeable shell encapsulating at leastone blowing agent. The shell is formed of a first polymeric layer thatsurrounds the blowing agent and a second polymeric layer that at leastsubstantially encapsulates the first polymeric layer. The firstpolymeric layer may be formed of polymers formed from nitrile containingmonomers, acrylic ester monomers, methacrylic ester monomers, vinylesters and/or vinyl halide monomers. The second polymeric layer includesat least one monomer that is either a monomer having a Tg of at least85° C. or a chemically reactive monomer. In one exemplary embodiment,the chemically reactive monomer provides functional groups to the secondlayer to permit the covalent bonding of a cationic species thereto. Inanother embodiment, the incorporation of high Tg monomers in the secondlayer creates an outer layer that has a greater concentration of thehigh Tg monomers compared to the first polymeric layer. The expandablemicrosphere may have a zeta potential that is greater than or equal tozero mV at a pH of about 9.0 or less at an ionic strength from 10-6 M to0.1 M. In addition, the microspheres may have a small expanded volumeaverage diameter, preferably less than about 20 μm.

It is another object of the present invention to provide a method offorming an expandable microsphere such as is described in the precedingparagraph. The method includes (1) mixing primary monomers selected fromnitrile containing monomers, acrylic ester monomers, methacrylic estermonomers, vinyl esters, vinyl halide monomers and combinations thereof,at least one blowing agent, a crosslinking monomer, a polymerizationinitiator, and a stabilizer in a reaction vessel for a period of timesufficient to achieve an approximate 90% polymerization of the primarymonomers and form a first polymeric layer surrounding the blowing agentand (2) adding a secondary monomer selected from monomers having a Tg ofat least 85° C. and chemically reactive monomers to the reaction vesselto form a second polymeric layer that at least substantially surroundsthe first polymeric layer and forms the expandable microsphere. Theaddition or “shot” of the high Tg monomers positions the high Tgmonomers in the second polymeric layer in a concentration that exceedsthe concentration of that particular Tg monomer in the first polymericlayer. In another embodiment, the chemically reactive monomer is addedas the “shot” of monomer to provide functional groups on the outersurface of the second layer and permit the covalent attachment ofcationic species. In at least one exemplary embodiment, a salt, a phasepartitioner, an inhibitor, an acid, and/or water may be added to thereaction vessel. In addition, the reaction vessel may be purged with aninert gas to remove unwanted oxygen in the headspace. It is a furtherobject of the present invention to provide a paper that includescellulose fibers and the expandable microspheres described above. Thepaper may have a Sheffield Smoothness of less than 250 SU as measured byTAPPI test method T 538 om-1 and/or a scanning 2nd cyan print mottle ofnot more than 6. In one embodiment, the paper may be calendared.

It is also an object of the present invention to provide a compositionthat includes the expandable microsphere described above and a pluralityof cellulose fibers.

It is an advantage of the present invention that a microsphere having aninner layer and an outer, functionalized layer may be formed bysuspension polymerization using a shot growth technique.

It is another advantage of the present invention that cations may becovalently and/or non-covalently bound to functional groups located onthe second (outer) layer of the microsphere to modify the surface chargeof the microsphere and create a modified microsphere.

It is yet another advantage of the present invention that a microspherehaving an inner layer and an outer layer including a monomer with a Tgof at least 85° C. may be formed by suspension polymerization using ashot growth technique.

The foregoing and other objects, features, and advantages of theinvention will appear more fully hereinafter from a consideration of thedetailed description that follows.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS OF THE INVENTION

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which the invention belongs. Although any methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of the present invention, the preferred methodsand materials are described herein. All references cited herein,including published or corresponding U.S. or foreign patentapplications, issued U.S. or foreign patents, or any other references,are each incorporated by reference in their entireties, including alldata, tables, figures, and text presented in the cited references.

It is to be appreciated that “thermally expandable microsphere”,“expandable microsphere”, and “microsphere” may be used interchangeablyherein. Additionally, the terms “first layer” and “second layer” may beinterchangeably used with “inner layer” and “outer layer”, respectively.The terms “polymer layer” and “polymeric layer” may also beinterchanged.

The present invention is directed to thermally expandable microspheresformed by suspension polymerization using a shot growth technique. Theshot growth method permits the formation of a second, outer layer thatincludes a chemically reactive monomer or an outer layer containing ahigh Tg monomer. The microspheres desirably have a cationic charge atthe outer surface to improve retention of the microspheres in thepapermaking process. In one or more exemplary embodiments, themicrospheres may have bound thereto a cationic species. The microspheresdescribed herein may be used in conventional papermaking processes toincrease bulk and to reduce the amount of cellulosic fiber used, therebyreducing manufacturing costs associated with the papermaking process.

The thermally expandable microspheres are formed of a continuousthermoplastic polymeric, gas impermeable shell housing therein at leastone blowing agent. In exemplary embodiments, the polymer shell of theexpandable microsphere has a first (inner) polymeric layer and a second(outer) polymeric layer. The second layer at least substantiallysurrounds or encapsulates the first layer. It is to be appreciated thatthe phrase “at least substantially surrounds” is meant to denote thatthe second layer surrounds or nearly surrounds the first layer.

As used herein, the term “layer” may be a separate, distinct, or aphysically separate portion of the microsphere. Also, a “layer” may notbe a separate, distinct, or a physically separate portion of themicrosphere. In one embodiment, two “layers” may have portions thereofthat interpenetrate and/or overlap with one another. In anotherembodiment, a “layer” may be the result of at least one of at least twopolymerization stages to create a continuous, polymeric shell from themonomers. In a further embodiment when at least two “layers” exist,there may be no clear demarcation of where the first “layer” ends andthe second “layer” begins, but rather a transition from one “layer” tothe other as a result of at least one first polymerization stage using aset of primary monomers at initial starting concentrations that areconsumed in part or in whole and at least one second polymerizationstage using a set of secondary monomer(s) at a second concentration(s),provided that the secondary monomer(s) is different than the primarymonomers or the second concentrations are different than the initialstarting concentrations.

The inner polymer layer may be formed of homo- and/or co-polymersobtained by polymerizing ethylenically unsaturated monomers. Inexemplary embodiments, the inner polymer layer is formed from thepolymerization and/or copolymerization of nitrile containing monomers,acrylic ester monomers, methacrylic ester monomers, vinyl esters, vinylhalide monomers, and combinations thereof. Suitable examples of nitrilecontaining monomers include acrylonitrile, methacrylonitrile,α-chloroacrylonitrile, α-ethoxyacrylonitrile, fumaronitrile, andcrotonitrile. Non-limiting examples of acrylic ester monomers includemethyl acrylate and ethyl acrylate. Examples of methacrylic estermonomers include, but are not limited to, methyl methacrylate, isobornylmethacrylate, gylcidyl methacrylate, tert-butylaminoethyl methacrylate,and ethyl methacrylate. Suitable examples of vinyl or vinylidene halidemonomers include vinyl chloride and vinylidene chloride. Non-limitingexamples of vinyl ester monomers include vinyl acetate and vinylpyridine.

In a preferred embodiment, the inner polymer layer is formed from thepolymerization and/or copolymerization of a nitrile containing monomer(e.g., acrylonitrile (AN)), an acrylic ester and/or methacrylic estermonomer (e.g., methyl methacrylate (MMA)), and a vinyl and/orvinylidenehalide monomer (e.g., vinylidene chloride (VDC)).

The vinyl and/or vinylidenehalide monomers may be present in the firstlayer in an amount of at least 50 weight % based on the total weight ofthe monomers. In exemplary embodiments, the vinyl and/orvinylidenehalide monomers may be present in an amount from about 55 toabout 95 weight %, from about 60 to about 90 weight %, and preferablyfrom greater than 65 to less than 85 weight %. As used herein, withrespect to the monomers, the phrase “weight %” is meant to denote“weight % based on the total weight of the monomers”. In addition, it isto be understood that all ranges recited herein are intended to includeall sub-ranges within the broad range.

The acrylic ester and/or methacrylic ester monomers may be present inthe first layer in an amount of at least 0.1 weight %. In exemplaryembodiments, the acrylic ester and/or methacrylic ester is present inthe first layer in an amount from about 0.5 to about 10 weight %, fromabout 1 to about 8 weight %, and preferably in an amount from about 1.5to about 5 weight %.

The nitrile containing monomers may be present in the first layer in anamount of at least 1 weight %. In one exemplary embodiment, the nitrilecontaining monomers may be present in the first layer in an amount fromabout 5 to about 40 weight %, from about 8 to about 35 weight %, andpreferably from about 10 to about 30 weight %.

The inner polymer layer may also include one or more multi-functionalcrosslinking monomers that have the ability to crosslink the monomerspresent within the first polymeric layer. Examples of suitablecrosslinking monomer include, but are not limited to, divinyl benzene,ethylene glycol di(meth)acrylate, di(ethylene glycol) di(meth)acrylate,triethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate,1,3-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate,glycerol di(meth)acrylate, 1,3-butanediol di(meth)acrylate, neopentylglycol di(meth)acrylate, 1,10-decanediol di(meth)acrylate,pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate,triallylformal tri(meth)acrylate, allyl methacrylate, trimethylolpropane tri(meth)acrylate, tributanediol di(meth)acrylate, PEG 200di(meth)acrylate, PEG 400 di(meth)acrylate, PEG 600 di(meth)acrylate,3-acryloyloxyglycol monoacrylate, triacryl formal, triallyl isocyanate,and triallyl isocyanurate. In at least one exemplary embodiment, thecrosslinking monomer is triallyl isocyanate. The crosslinking monomer(s)may be present in the first layer in an amount of at least 0.05 weight%. In one or more exemplary embodiments, the crosslinking monomers arepresent in an mount from about 0.07 to 5 weight %, from 0.1 to 4 weight%, and preferably, from 0.2 to 3 weight % based on the total weight ofthe monomers.

Additionally, the monomers forming the first layer (primary monomers),including the crosslinking monomers, may be present in amounts such thatthe weight average glass transition temperature (Tg) of the monomers (asif the monomers were in their mono-polymerized form) is less than 100°C., desirably less than 95° C., more desirably less than 90° C., andmost desirably less than 85° C. The first polymer layer may also haveany net charge, but desirably possesses a net negative charge. Also, themonomers of the first layer desirably have a weight average Tg that isless than the weight average Tg of the monomers of the outer layer. Asan example only, if the first layer were to contain 3 wt % methylmethacrylate (Tg of PMMA is 105° C.), 17 wt % acrylonitrile (Tg of PANis 95° C.) and 80 wt % of vinylidene chloride (Tg of PVC is 81° C.),then the weight average Tg of the monomers of the first polymeric layeraccording to this exemplified embodiment is 84.45° C.

In addition, it is possible that the monomers forming the first layer(primary monomers), including the crosslinking monomers, may be presentin the outer layer, albeit at different concentrations and ratios ofsuch monomers.

In one exemplary embodiment, the outer layer includes monomers(secondary monomers) that have a Tg of at least 85° C., preferably atleast 90° C., and even more preferably, 95° C. In particular, the secondlayer may be formed of homopolymers and/or copolymers comprised of thesehigh Tg monomers. As discussed in detail below, the addition of high Tgmonomers near the end of the polymerization of the primary monomerscreates an outer layer having a high concentration of the high Tgmonomers, especially when compared to the first layer. This outer(second) layer containing the high Tg monomers both improves the thermalresistance and increases the strength of the microspheres. Non-limitingexamples of suitable high Tg monomers include acrylonitrile (AN)monomers, vinylidene chloride (VDC), methyl methacrylate (MMA) monomers,tetraethylene glycol dimethacrylate monomers (TEGDMA), 2,3-epoxypropylacrylate monomers (EPA) and methacrylonitrile monomers (MAN). In atleast one exemplary embodiment, acrylonitrile is used as the high Tgmonomer to create a second layer formed of (poly)acrylonitrile (PAN).

In another exemplary embodiment, the outer layer includes chemicallyactive monomers may contain functional groups that remain chemicallyactive after polymerization and present such functional groups on theoutside surface of the shell. Accordingly, these chemically activemonomers are able to generate surface onium ions and may also possessthe ability to crosslink and improve the strength of the outer layer. Inone or more embodiments, microspheres functionalized with GMA or VBC maybe reacted with a nucleophile (e.g., 0.16 g of dimethyl sulfide or 0.62g of trimethylamine) over a period of 24 hours at 60° C. to convert thesurface functional groups to the cationic sulfonium and ammonium forms(as is disclosed, for example, for emulsion polymerization in U.S. Pat.Nos. 4,056,501; 4,002,586; and 3,936,890.

The incorporation of functional monomers in the outer layer may improveparticle strength by crosslinking the surface of the microspheres. Thefunctional groups on the monomers have covalently bonded thereto acationic species. The cationic species is advantageously not desorbedwhen the microspheres are re-dispersed into large volumes of water, evenwhen the microspheres are dispersed in the water for extended periods oftime. The reactive monomers may include glycidyl methacrylate (GMA),methacrylic acid (MAA), vinyl benzyl chloride (VBC), and combinationsthereof. In at least one exemplary embodiment, the cationic species isan organic (onium-type) cation, such as, for example, sulfonium andammonium cations.

The combined net zeta potential of the first and second polymeric layersmay be negative, neutral, or positive as measured by the net zetapotential determined at a pH of about 9.0 or less at an ionic strengthfrom about 10⁻⁶ M to 0.1 M. In one embodiment, the net zeta potential ofthe polymeric shell may be from −100 to +500 mV. Preferably, the netzeta potential is from greater than or equal to zero to +500 mV. Inexemplary embodiments, the net zeta potential is from greater than orequal to zero to +150 mV, most preferably from +10 to +130 mV at a pH ofabout 9.0 or less at an ionic strength of from 10⁻⁶ M to 0.1 M asmeasured by standard and conventional methods of measuring zetapotential known to those of skill in the analytical and physical arts,preferably methods utilizing microelectrophoresis at room temperature.It is to be appreciated that the microsphere may also possess theabove-described zeta potentials. U.S. Patent Publication No.2007/0044929, which is incorporated by reference in its entirety,describes an additional embodiment to for modifying the charge of themicrosphere via a covalent attachment.

The blowing agent within the microsphere is not particularly limited,and may be any blowing agent that, upon the application of heat energy,functions to provide internal pressure on the shell of the microsphereto force the microsphere to expand. The blowing agent may be liquidand/or gas. Non-limiting examples of suitable blowing agents for use inthe microsphere include low boiling point hydrocarbons (e.g., propane,n-pentane, isopentane, neopentane, hexane, neohexane, butane,isoheptane, octane, and isooctane) and/or chlorinated hydrocarbons orfluorinated hydrocarbons (e.g., methyl chloride, methylene chloride,dichloroethane, dichloroethane, trichloroethane, and perfluorinatedhydrocarbons). One or more blowing agent may be present within themicrosphere. In exemplary embodiments, the blowing agent is isopentaneor n-butane.

The expandable microspheres may contain any suitable amount of blowingagent. In at least one exemplary embodiment, the microspheres maycontain at least 5% and not more than 50 wt % of the blowing agent basedon the total weight of the blowing agent and the monomers containedwithin the polymer shell. In other exemplary embodiments, the blowingagent is present in the microsphere in an amount from about 10 to about45 wt %, and preferably from about 20 to about 40 wt %.

The first and second layers of the microsphere may be the same, but aredesirably not chemically and/or physically equivalent. Therefore, evenwhen the first and second layers contain similar, if not identicalmonomers, the physical properties of the layers may be different due todifferences in the ratios of monomers contained in each layer. Further,the existence of additive monomers, such as crosslinking monomers, mayalso create chemical and physical differences between the inner andouter layers of the expandable microsphere.

The properties of interest for the expandable microsphere include thetemperature onset of expansion (To.e), the temperature onset ofshrinkage (To.s), the maximum expansion volume, and the particle size.The temperature at which the expandable microsphere begins to expand isconsidered the temperature onset of expansion (To.e). It may also beknown as Tstart. Desirably, a maximum expansion volume and narrow To.e,To.s, and particle size are achieved in the inventive microspheres. TheTo.e and the To.s may be determined by hot stage microscopy or with anoptical microscope equipped with a heating stage. For example, a samplecontaining the microspheres may be heated at a rate of 10° C. per minuteand the temperature onset of expansion and temperature onset ofshrinkage visually noted. The microspheres may have a To.e. from about60 to about 105° C., from about 65 to about 100° C., from about 70 toabout 98° C., and preferably from about 75 to about 95° C. and a To.sthat is greater than about 105° C., greater than about 110° C., greaterthan about 120° C., and preferably greater than about 130° C.

The maximum expansion volume of the particles at 110° C., denoted asV₁₁₀, may be determined by placing 0.2 g of dried particles in a 50 mltest tube. The test tube was then immersed in an oil bath maintained at110° C. The maximum expanded volume was recorded by comparing the volumeof the expanded particles in the test tube to a graduated (calibrated)test tube of the same size. The particle size distributions weremeasured using a Horiba LA-910 Laser Light Scattering Analyzer. Themaximum expansion volume at 110° C. (V₁₁₀) of a the microspheres may befrom 5 to 50 ml, from 7 to 40 ml, from 8 to 35 ml, and preferably from10 to 30 ml.

The maximum expansion temperature, Tmax, can be determined by usingthermomechanical analysis (TMA; Model 2940; TA Instruments). TMA may bemeasured to probe the expansion of particles under pressure load. 5 mgof unexpanded particles are deposited on an aluminum sample pan evenlyand covered with a flat lid to prevent them from sticking to theexpansion probe or sliding out the sides of the sample pan duringexpansion. The sample is placed under a pressure load of 1 N and heatedfrom 60 to 150° C. at 5° C. per minute. The Tmax is the temperature atwhich the maximum expansion occurs. The expandable microspheres may haveany Tmax, but desirably has a Tmax from about 90 to about 100° C., fromabout 95 to about 135° C., or from about 100 to about 120° C.

It is believed that the microspheres have an improved structuralintegrity by maintaining the blowing agent inside the shell and reducingshrinkage at low temperatures. As a result, an improved To.e and To.sprofile is achieved that is more conducive to papermaking, which leadsto more efficient bulking in paper at lower doses of microspheres in thepaper.

The microspheres may have a volume average diameter in an expanded statefrom about 1 μm to about 100 μm, preferably from about 1 μm to about 50μm, and more preferably from about 5 μm to about 40 μm, or from about 10μm to about 20 μm. In exemplary embodiments, the microspheres have anexpanded volume average diameter less than about 50 μm, less than about30 μm, less than about 20 μm, or less than about 17 μm. The expandedvolume average diameter as used herein refers to values obtained bymeasuring the microspheres according to ISO 13319:2000, “Determinationof Particle Size Distributions—Electrical Sensing Zone and Method”.

In addition, the microspheres may have a maximum expansion from about 1to about 15 times, preferably from about 1.5 to 10 times, and morepreferably from about 2 to 5 times the mean diameters. The microspheresmay also have a maximum expansion volume from about 1 to about 100times, preferably from about 5 to about 50 times, and even morepreferably from about 10 to about 35 times the initial volume (i.e.,unexpanded volume).

The expandable microspheres of the present invention may be made in anymanner. One inventive method embodiment is discussed in detail below. Inthis embodiment, the microspheres are formed by suspensionpolymerization utilizing a “shot growth” technique. “Shot growth” may bedefined herein as a process whereby polymerization of a first set ofmonomers (primary monomers) occurs until the first set of monomers issubstantially consumed and a quantity or “shot” of a second monomer isadded to drive the polymerization reaction to completion.

In one embodiment, the microspheres may have an expanded volume averagediameter on the order of about 20 μm, and in one embodiment, less than20 μm, are able to be produced. Although not wanting to be bound by anyparticular theory, it is hypothesized that the stabilizer systemsufficiently stabilizes the polymer dispersion to permit adequatecontrol of the polymerization and degree of polymerization and toprevent an agglomeration of the microspheres so as to achievemicrospheres having diameters that are optimal for papermaking. Withoutsufficient stabilization, it has been determined that the microspherestend to agglomerate or grow together, and control of the polymerizationcannot be achieved.

As the primary monomers coalesce and form polymers, the inner layer ofthe microsphere takes form and thickens as the polymerization reactionprogresses. After a period of time, the initial set of monomers, beingprovided at desired concentrations, are consumed in whole or in part andthe energy from the polymerization reaction decreases. At this point inthe process, an amount or “shot” of an additional monomer (secondarymonomer) is added to the reaction system to drive the reaction tocompletion or nearly complete. A multitude of secondary monomers can beadded to by this shot growth method to control the morphology andexpandability of the microspheres. Also the monomer “shot” allows for aspecific tailoring of the microsphere for desired purposes, such as, forexample, papermaking.

Additionally, the shot growth method provides a way for the growth ofthe monomer to yield properties for strength and/or functionality. Forinstance, a high Tg monomer can be added as the secondary or “shotgrowth” monomer to position the high Tg monomer in the outer layer in aconcentration that greatly exceeds the concentration of that particularTg monomer in the inner layer. Alternatively, a chemically reactivemonomer may be added as the shot-growth monomer to provide functionalgroups on the outer surface and permit the covalent attachment ofcationic species. The combination of monomers forming the microsphere ischosen so as to achieve a desired performance level, optimal onset oftemperature of shrinkage, optimal temperature onset of expansion,optimal maximum expansion temperature, and optimal distribution ofparticles for use in papermaking to increase the bulk of the paper.

The “shot growth” polymerization forms microspheres having an inner(i.e., first) polymeric layer and an outer (i.e., second) polymer layer.To form a microsphere according to the invention, at least one organicphase is contacted with at least one aqueous phase. The aqueous phaseand the organic phase may be mixed to form a suspension and/ordispersion, and/or a system in which the organic phase droplets areformed in the aqueous phase.

The organic phase may contain any of the above-mentioned monomers in anyamounts suitable to achieve expandable microspheres having the monomericcompositions in the above-described amounts and/or any one or more ofthe above-described properties and/or any one or more of theabove-described characteristics. Although any of the above-describedmonomers for use in the first polymer layer may be used in accordancewith the invention, reference will be made herein to a preferredembodiment in which vinylidene chloride (VDC), acrylonitrile (AN), andmethyl methacrylate (MMA) are the primary monomers.

In one embodiment the primary monomers may be added to the reactionmixture such that the vinyl and/or vinylidene halide monomers are addedat an amount not more than 60 wt % based upon the total weight of thereaction mixture, the nitrile containing monomers are added at an amountnot more than 10 wt % based upon the total weight of the reactionmixture, the acrylic ester and/or methacrylic ester monomers are addedat an amount not more than 2 wt % based upon the total weight of thereaction mixture.

In one embodiment, it has been discovered that microspheres with higherVDC levels demonstrate increased expanded volumes due to plasticizationby residual monomer in addition to the polymer being more plastic.

The organic phase may also contain a polymerization initiator. Thepolymerization initiator is any chemical or compound that is capable ofgenerating free radicals when the organic and aqueous phases arecombined. The polymerization initiator may be added at any suitableamount to drive the polymerization of the polymer shell to completenessand deplete nearly all of the monomers present in the organic phase.Non-limiting examples of suitable polymerization initiators include2,2′-azobisisobutyronitrile. Additionally, a crosslinking monomer (e.g.,triallyl cyanurate (TAC) and a blowing agent (e.g., n-butane orisobutane) are included in the organic phase.

The aqueous phase also includes water, preferably deionized water, and astabilizer or a stabilizer system which functions to stabilize theorganic phase droplets. In one or more exemplary embodiments, thestabilizer system includes a stabilizer and a polyelectrolyte.Non-limiting examples of stabilizers for use in the aqueous phaseinclude a silicon-containing compound, an aluminum-containing compound,colloidal silica, alumina, and colloidal alumina. The polyelectrolytemay be a cationic polyelectrolyte such as polyvinylamine (PVAm), suchas, but not limited to, those commercially available from BASF under thetrade name Lupamin® (e.g., Lupamin® 5095). Although not wishing to bebound by any particular theory, it is believed that the polyeletrolytesact as a promoter of the flocculation of the stabilizer, thereby drivingthe stabilizer to the organic phase/aqueous phase interface and/ordroplet surface to more effectively stabilize the droplet. Accordingly,it is believed that the polyvinylamine forms aggregates with the silicaparticles to allow for better adsorption onto the monomer droplets. Inat least one exemplary embodiment of the invention, the stabilizersystem includes colloidal silica and polyvinylamine.

Additionally, a salt may be included in the aqueous phase to adjust theelectrolyte concentration in the aqueous phase so that thehydrophobicity of the stabilizer is maintained at an elevated level. Thesalt is not particularly limited, and may be chosen from sodiumchloride, calcium chloride, and/or aluminum chloride. In exemplaryembodiments, sodium chloride (NaCl) is the salt utilized in the aqueousphase.

A phase partitioner such as ethanol may also be included in the aqueousphase to lower the interfacial tension between the oil and the aqueousphases and improve the phase separation of the blowing agent. Inaddition, sodium dichromate, or other suitable inhibitor, may be addedas an inhibitor to inhibit, or even prevent, polymerization in theaqueous phase.

The aqueous phase may have any pH so long as the pH is at or near theisoelectric point of the stabilizer, which increases the stabilizerstabilizer's effectiveness to stabilize the organic droplet when theorganic and aqueous phases are mixed. Desirably, the pH is an acidic pH.In exemplary embodiments, the pH is less than 5, and preferably lessthan 4. In at least one exemplary embodiment, the pH is 3.5. The pH ofthe aqueous phase may be pre-adjusted, i.e., prior to mixing with theorganic phase. In addition, the pH may be adjusted with any suitableacid, such as hydrochloric acid.

The components of the organic and aqueous phase according to oneexemplary embodiment are depicted in Table 1.

TABLE 1 Organic Phase Components Aqueous Phase Components Monomers:Deionized Water vinylidene chloride acrylonitrile methyl methacrylateInitiator Stabilizer System 2,2′-azobisisobutyronitrile colloidal silicapolyvinyl amine Crosslinking Monomer Sodium Chloride tallyl cyanurateBlowing Agent Ethanol n-butane isobutane Sodium Dichromate HydrochloricAcid

The organic phase and the aqueous phase are mixed, preferably at atemperature below the boiling points of the blowing agent(s). In oneexemplary embodiment, the organic phase and the aqueous phase arehomogenized in one vessel (e.g., a homogenizer or mixer) and thentransferred and sealed in reactor vessel (e.g., pressure reactionbottle). The homogenization process forms the initial organic phasedroplets. Thus, it is to be appreciated that variations in thehomogenization process results in different droplet sizes in thesuspensions and different particle sizes after polymerization.

In one embodiment, the reactor bottles are purged with an inert gas(e.g., argon) to remove oxygen from the headspace before adding thesuspension. It was surprisingly discovered that oxygen, if is present inthe reactor vessel, acts as an inhibitor, and polymerization of themonomers will not begin until the oxygen is consumed. In addition, thepolymerization rate of the monomers can be significantly slower due tothe lower initiator level (the oxygen consumes some of the initiatorradicals). A lower polymerization rate results in a coagulation of thesuspension as a result of the particles polymerizing too slowly andcapturing each other during the course of the polymerization. Also, whenoxygen is present, the final microspheres may undesirably bemulti-cellular and the particle size distribution may be broader due tooxygen-induced aggregation.

After synthesis of the microspheres, the microspheres are filtered,washed, preferably with deionized water, and dried. The driedmicrospheres contain a residual web of silica that needs to be removedprior to expansion if, for example a lower vapor pressure blowing agentsuch as isobutane is utilized as the blowing agent. To remove the silicashell, the dried microspheres may be dispersed in deionized water andhydrogen peroxide. The resulting slurry may then be mixed and/oragitated (e.g., tumbled in the reaction vessel) to break the silicashell. Once the hydrogen peroxide treatment is complete, themicrospheres are again washed to remove the silica particles, and dried.

In a separate embodiment, the organic and aqueous phases are mixed in asingle reactor vessel (e.g. pressure reactor) under pressure (preferablypurged with argon). The entire suspension is sheared in the same reactorvessel for a period of time until homogenization is complete. Thus,unlike the previously described embodiment, there is no transfer of thereaction mixture after homogenization. Additionally, in this exemplaryembodiment, n-butane may be used as the blowing agent. The use ofn-butane eliminates the need to remove residual silica from themicrospheres by a hydrogen peroxide treatment. Because the n-butane hasa high vapor pressure (one that is more than double the vapor pressureof isobutane), n-butane is able to expand the microspheres even whenresidual silica is still present on the surface of the microsphere. Onthe other hand, microspheres formed with lower vapor pressure blowingagents such as isobutane, a web of silica covering the microspheres mustbe broken with a hydrogen peroxide treatment and removed from theparticle surface.

After the homogenized suspension is sealed in the reactor vessel, thesuspension is heated, preferably with continuous mixing and at a lowershear rate, for a period of time until the polymerization reaction isnearly complete and/or the initiator is exhausted or depleted, yetadditional monomers exist in the reaction mixture. In one exemplaryembodiment, the polymerization reaction occurs until the polymerizationof the primary monomers is approximately 90% complete. The period oftime for the polymerization of the monomers varies in accordance withthe desired properties of the polymeric shell and the reactionconditions and/or parameters. The period of time may be less than orequal to 24 hours, less than or equal to 17 hours, or less than 12hours. In exemplary embodiments, the polymerization reaction time is notmore than 6 hours or 8 hours.

After the polymerization of the monomers is nearly complete (e.g.,approximately 90% complete) and/or the initiator has been depleted, a“shot” of additional monomer and/or initiator is added to thepolymerization reactor to drive the reaction to completeness and also toform the second polymer layer. With this addition, the formation of thefirst polymer layer is complete and the formation of the second polymerlayer begins. The addition of a secondary monomer (with or withoutinitiator) permits the initiation of the second polymer layer becausethe secondary monomers tend to polymerize on the surface of the firstpolymer layer. Any of the above-mentioned monomers may be used as thesecondary monomers, and the secondary monomer(s) may be the same ordifferent from the monomers present in the first polymer layer. As aresult, the second polymer layer may have similar chemical and physicalcharacteristics as that of the first polymer layer. Desirably, thesecond layer has chemical and/or physical characteristics that aredifferent from the first polymer layer.

Various monomers may be added by shot growth to control the morphologyand expandability characteristics of the microsphere. Additionally, the“shot” of monomer(s) to the reaction vessel may also be used to increasethe concentration of a particular monomer at the particle surface.According to one embodiment, at least one monomer having a high Tg(e.g., a Tg of at least 85° C.) is added to the reaction vessel as the“shot” of monomer. As discussed above, the Tg of the monomer isdesirably at least 90° C., and even more desirably, 95° C. Any one ofthe high Tg monomers described above may be used to form the secondlayer.

As one example, 2,2′-azobisisobutyronitrile dissolved in acrylonitrile(AN) may be added to the reaction vessel as the initiator “shot” in theshot growth method. Because the monomer reacts primarily at the surfaceof the microsphere, relatively low amounts of monomer can be added andstill create high levels of surface incorporation. The inclusion ofadditional acrylonitrile to the reaction vessel forms a second layerrich in poly(acrylonitrile) at the surface of the microspheres. It isbelieved that the double bonds present in the acrylonitrile monomerscreate a tight and robust microsphere that has improved strengthcharacteristics. In addition to improving the strength of themicrosphere, the addition of a high Tg monomer improves the thermalresistance of the microspheres, increased expansion volume, and higheronset temperature of shrinkage. Further, the high Tg monomer creates asecond layer that is stronger in comparison to the first layer.

In another embodiment, the shot growth technique may be used tofunctionalize the particle surface. In this embodiment, monomers thatremain chemically reactive after polymerization may be added to thereaction vessel. Non-limiting examples of such monomers include, but arenot limited to, glycidyl methacrylate (GMA), methacrylic acid (MAA),vinyl benzyl chloride (VBC), and combinations thereof. As with theembodiment described above, the monomers react primarily at the surfaceof the microsphere (i.e., the surface of the first layer), causing thereactive functional groups of the monomer to be present on the outersurface of the second layer. It is to be appreciated that themicrosphere, at this stage, has a negative charge.

Next, the functional groups on the surface of the second layer may bereacted with a cation, such as an organic (onium-type) cation, to form apositively charged microsphere surface that is relatively more cationicthan conventional microspheres. Examples of onium-type cations includesulfonium and ammonium cations. The cation reacts with the functionalgroup on the monomer to covalently bond the cation to the monomer. Theresulting functionalized microspheres are relatively more cationic thanconventional microspheres. As a result, such microspheres are able to beretained in the web of fibers more readily when being used in papermaking, thereby enhancing the efficiency of such microspheres byenabling the papermaker to use less microspheres at a desired bulk.

The “shot” of monomer(s) may be added as early as 6 hours into thepolymerization of the primary monomers. In exemplary embodiments, theshot growth, and subsequent polymerization of the secondary monomer(s),occurs at 8, 12 or 17 hours after the initiation of the polymerizationof the primary monomers.

In one embodiment, the “shot” of monomer(s) may be added such that theamount of the high Tg monomer or the chemically reactive monomer rangesfrom about 0.2 wt % to 5 wt % based upon the total weight of theoriginal reaction mixture. The amount of the shot” of monomer(s) may beadded such that the amount of the high Tg monomer or the chemicallyreactive monomer is at least 0.3, 0.5, 0.75, 1.0, 1.25, 1.5, 1.75, 2.0,2.25, 2.5, 2.75, 3.0, 3.25, 3.5, 3.75, 4.0, 4.25, 4.5, 4.75, and 5 wt %based upon the total weight of the reaction mixture, including any andall ranges and subranges therein.

In one embodiment, the “shot” of monomer(s) may also contain otherreactants in the original reaction mixture. In one embodiment, the“shot” contains an initiator that may be identical to, or differentfrom, the initiator used in the original reaction mixture. Within thisshot, the initiator may be added such that the amount of the initiatorranges from about 25 wt % to 200 wt % based upon the total weight of theamount of initiator used in the original reaction mixture. Theadditional initiator may be added via the “shot” such that the amount ofthe initiator is 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90,95, 100, 110, 120, 130, 140, 150, 175, and 200 wt % based upon the totalweight of the amount of initiator used in the original reaction mixture,including any and all ranges and subranges therein.

While the microspheres of the present invention may be or have any use,an exemplary use is in conventional papermaking processes to make paper.The paper includes a web of cellulose fibers and the above-describedmicrospheres. The term “paper” as used herein is meant to include alltypes of cellulose-based products in sheet or web form, including, butnot limited to, paper, paperboard, paper substrates, and cardboard. Asused herein, the terms “paper, “paper substrate”, and “substrate” may beinterchangeably used. The paper may be produced as a single layer or amulti-layer paper having two or more layers. Additionally, the paper mayor may not be calendered. Paper according to the present invention maycontain from 1 to 99 wt %, and desirably from 5 to 95 wt % of cellulosefibers based upon the total weight of the paper.

The amount of microspheres present in the paper depends upon the totalweight of the substrate and/or the final paper or paperboard product.The paper substrate may contain greater than 0.001 wt %, more preferablygreater than 0.02 wt %, most preferably greater than 0.1 wt % of themicrospheres based on the total weight of the substrate. Further, thepaper substrate may contain less than 20 wt %, preferably less than 10wt %, and more preferably less than 5 wt % of the microspheres based onthe total weight of the substrate.

The paper may contain recycled fibers and/or virgin fibers. Recycledfibers differ from virgin fibers in that the fibers have gone throughthe drying process at least once. In certain embodiments, at least aportion of the cellulose/pulp fibers may be provided from non-woodyherbaceous plants including, but not limited to, kenaf, hemp, jute,flax, sisal, or abaca, although legal restrictions and otherconsiderations may make the utilization of hemp and other fiber sourcesimpractical and/or impossible. Either bleached or unbleached pulp fibermay be utilized. Desirably, the sources of the cellulose fibers are fromsoftwood and/or hardwood.

Further, the softwood and/or hardwood fibers contained by the paper maybe modified by physical and/or chemical means. Examples of physicalmeans include, but are not limited to, electromagnetic and mechanicalmeans. Non-limiting means for electrical modification include meansinvolving contacting the fibers with an electromagnetic energy sourcesuch as light and/or electrical current. Suitable means for mechanicalmodification include means involving contacting an inanimate object withthe fibers. Examples of such inanimate objects include those with sharpand/or dull edges. Such means also involve, for example, cutting,kneading, pounding, and/or impaling.

Examples of chemical means include conventional chemical fibermodification means such as crosslinking and precipitation of complexesthereon. Examples of such modification of fibers may be such as thosefound in the following U.S. Pat. Nos. 6,592,717; 6,592,712; 6,582,557;6,579,415; 6,579,414; 6,506,282; 6,471,824; 6,361,651; 6,146,494;H1,704; 5,731,080; 5,698,688; 5,698,074; 5,667,637; 5,662,773;5,531,728; 5,443,899; 5,360,420; 5,266,250; 5,209,953; 5,160,789;5,049,235; 4,986,882; 4,496,427; 4,431,481; 4,174,417; 4,166,894;4,075,136; and 4,022,965, which are hereby incorporated by reference intheir entireties. Further modification of fibers may be found in U.S.Patent Application Ser. No. 60/654,712 filed Feb. 19, 2005 (incorporatedby reference in its entirety), which modification may include theaddition of optical brighteners (i.e. OBAs) as discussed therein.

In a preferred embodiment, any of the above-mentioned fibers may betreated so as to have a high ISO brightness. Examples of fibers treatedin this manner include, but are not limited to, those fibers describedin U.S. patent application Ser. No. 11/358,543 filed Feb. 21, 2006entitled “PULP AND PAPER HAVING INCREASED BRIGHTNESS”, which is herebyincorporated by reference in its entirety; and PCT Patent ApplicationNumber PCT/US06/06011 filed Feb. 21, 2006 entitled “PULP AND PAPERHAVING INCREASED BRIGHTNESS”, which is hereby incorporated by referencein its entirety.

While the pulp, fibers, and/or paper may have any brightness and/or CIEwhiteness, preferably such brightness and/or CIE whiteness is asfollows. The fiber and/or the pulp and/or paper substrate according toembodiments of the present invention may have any CIE whiteness, butpreferably have a CIE whiteness greater than 70, more preferably greaterthan 100, even more preferably greater than 125, or even greater than150. The CIE whiteness may be in the range from 125 to 200, preferablyfrom 130 to 200, and most preferably from 150 to 200. Examples ofmeasuring CIE whiteness and obtaining such whiteness in a fiber andpaper made therefrom can be found, for example, in U.S. Pat. No.6,893,473, which is hereby incorporated by reference in its entirety.

The fibers, the pulp and/or paper may have any ISO brightness, butpreferably greater than 80, more preferably greater than 90, mostpreferably greater than 95 ISO brightness points. The ISO brightness maybe preferably from 80 to 100, more preferably from 90 to 100, mostpreferably from 95 to 100 ISO brightness points. Examples of measuringISO brightness and obtaining such brightness in a papermaking fiber andpaper made therefrom can be found, for example, in U.S. Pat. No.6,893,473, which is hereby expressly incorporated by reference in itsentirety.

The paper substrate according to the present invention may be made offof the paper machine having any basis weight. The paper substrate mayhave either a high or low basis weight, including basis weights of atleast 10 lbs/3000 square foot, preferably from at least 20 to 500lbs/3000 square foot, and more preferably from at least 40 to 325lbs/3000 square foot. Of course, one of skill will appreciate that theseweights can easily be converted so as to be based upon 1300 square foot.

The paper substrate according to the present invention may have aSheffield Smoothness of less than 400 Sheffield Units (SU). However, thepreferred Sheffield Smoothness will be driven by the end product papersubstrate's intended use. Preferably, the paper has a SheffieldSmoothness of less than 350 SU, preferably less than 250 SU, and morepreferably less than 200 SU, as measured by TAPPI test method T 538om-1.

The paper may include optional substances such as, but not limited to,retention aids, sizing agents, binders, fillers, thickeners, andpreservatives. Examples of fillers include, but are not limited to,clay, calcium carbonate, calcium sulfate hemihydrate, and calciumsulfate dehydrate. A preferred filler is calcium carbonate with thepreferred form being precipitated calcium carbonate. Non-limitingexamples of binders include polyvinyl alcohol, Amres (a Kymene type),Bayer Parez, polychloride emulsion, modified starch such as hydroxyethylstarch, starch, polyacrylamide, modified polyacrylamide, polyol, polyolcarbonyl adduct, ethanedial/polyol condensate, polyamide,epichlorohydrin, glyoxal, glyoxal urea, ethanedial, aliphaticpolyisocyanate, isocyanate, 1,6 hexamethylene diisocyanate,diisocyanate, polyisocyanate, polyester, polyester resin, polyacrylate,polyacrylate resin, acrylate, and methacrylate. Other optionalsubstances include, but are not limited to, silicas such as colloidsand/or sols. Suitable examples of silicas include, but are not limitedto, sodium silicate and/or borosilicates. Other examples of optionalsubstances are solvents such as water.

The paper substrate of the present invention may also contain retentionaids selected from coagulation agents, flocculation agents, andentrapment agents dispersed within the bulk and porosity enhancingadditives cellulosic fibers, such as the microspheres of the presentinvention. Retention aids for the bulk-enhancing additives retain asignificant percentage of the additive in the middle of the paperboardand not in the periphery. Suitable retention aids function throughcoagulation, flocculation, or entrapment of the bulk additive.Coagulation comprises a precipitation of initially dispersed colloidalparticles. This precipitation is suitably accomplished by chargeneutralization or formation of high charge density patches on theparticle surfaces. Since natural particles such as fines, fibers, clays,etc., are anionic, coagulation is advantageously accomplished by addingcationic materials to the overall system. Such selected cationicmaterials suitably have a high charge to mass ratio. Suitable coagulantsinclude inorganic salts such as alum or aluminum chloride and theirpolymerization products (e.g. PAC or poly aluminum chloride or syntheticpolymers); poly(diallyldimethyl ammonium chloride) (i.e., DADMAC);poly(dimethylamine)-co-epichlorohydrin; polyethylenimine;poly(3-butenyltrimethyl ammoniumchloride);poly(4-ethenylbenzyltrimethylammonium chloride);poly(2,3-epoxypropyltrimethylammonium chloride);poly(5-isoprenyltrimethylammonium chloride); andpoly(acryloyloxyethyltrimethylammonium chloride). Other suitablecationic compounds having a high charge to mass ratio include allpolysulfonium compounds, such as, for example, the polymer made from theadduct of 2-chloromethyl; 1,3-butadiene and a dialkylsulfide, allpolyamines made by the reaction of amines such as, for example,ethylenediamine, diethylenetriamine, triethylenetetraamine or variousdialkylamines, with bis-halo, bis-epoxy, or chlorohydrin compounds suchas, for example, 1-2 dichloroethane, 1,5-diepoxyhexane, orepichlorohydrin, all polymers of guanidine such as, for example, theproduct of guanidine and formaldehyde with or without polyamines. Thepreferred coagulant is poly(diallyldimethyl ammonium chloride) (i.e.,DADMAC) having a molecular weight of about ninety thousand to twohundred thousand and polyethylenimene having a molecular weight of aboutsix hundred to 5 million. The molecular weights of all polymers andcopolymers herein this application are based on a weight averagemolecular weight commonly used to measure molecular weights of polymericsystems.

Another advantageous retention system suitable for the manufacture ofpaperboard is flocculation, which is essentially the bridging ornetworking of particles through oppositely charged high molecular weightmacromolecules. Alternatively, the bridging may be accomplished byemploying dual polymer systems. Macromolecules useful for the singleadditive approach include cationic starches (both amylase andamylopectin), cationic polyacrylamide such as for example,poly(acrylamide)-co-diallyldimethyl ammonium chloride;poly(acrylamide)-co-acryloyloxyethyl trimethylammonium chloride,cationic gums, chitosan, and cationic polyacrylates. Naturalmacromolecules such as, for example, starches and gums, are renderedcationic usually by treating them with 2,3-epoxypropyltrimethylammoniumchloride, but other compounds can be used such as, for example,2-chloroethyl-dialkylamine, acryloyloxyethyldialkyl ammonium chloride,acrylamidoethyltrialkylammonium chloride, etc. Dual additives useful forthe dual polymer approach are any of those compounds which function ascoagulants plus a high molecular weight anionic macromolecule such as,for example, anionic starches, CMC (carboxymethylcellulose), anionicgums, anionic polyacrylamides (e.g., poly(acrylamide)-co-acrylic acid),or a finely dispersed colloidal particle (e.g., colloidal silica,colloidal alumina, bentonite clay, or polymer micro particles marketedby Cytec Industries as Polyflex). Natural macromolecules such as, forexample, cellulose, starch and gums are typically rendered anionic bytreating them with chloroacetic acid, but other methods such asphosphorylation can be employed. Suitable flocculation agents arenitrogen containing organic polymers having a molecular weight of aboutone hundred thousand to thirty million. The preferred polymers have amolecular weight of about ten to twenty million. The most preferred havea molecular weight of about twelve to eighteen million. Suitable highmolecular weight polymers are polyacrylamides, anionicacrylamide-acrylate polymers, cationic acrylamide copolymers having amolecular weight of about five hundred thousand to thirty million andpolyethylenimenes having molecular weights in the range of about fivehundred thousand to two million.

The third method for retaining the bulk additive in the paper isentrapment. This is the mechanical entrapment of particles in the fibernetwork. Entrapment is suitably achieved by maximizing network formationsuch as by forming the networks in the presence of high molecular weightanionic polyacrylamides, or high molecular weight polyethyleneoxides(PEO). Alternatively, molecular nets are formed in the network by thereaction of dual additives such as, for example, PEO and a phenolicresin.

The optional substances may be dispersed throughout the cross section ofthe paper substrate or they may be more concentrated within the interiorof the cross section of the paper substrate. Further, other optionalsubstances such as binders and/or sizing agents (for example) may beconcentrated more highly towards the outer surfaces of the cross sectionof the paper substrate. More specifically, a majority percentage ofoptional substances such as binders or sizing agents may preferably belocated at a distance from the outside surface of the substrate that isequal to or less than 25%, more preferably 10%, of the total thicknessof the substrate. Examples of localizing such optional substances suchas binders/sizing agents as a function of the cross-section of thesubstrate is, for example, paper substrates having an “I-beam” structureand may be found in U.S. Provisional Patent Application Ser. No.60/759,629 entitled “PAPER SUBSTRATES CONTAINING HIGH SURFACE SIZING ANDLOW INTERNAL SIZING AND HAVING HIGH DIMENSIONAL STABILITY”, which ishereby incorporated by reference in its entirety. Further examples thatinclude the addition of bulking agents may be found in U.S. ProvisionalPatent Application Ser. No. 60/759,630 entitled “PAPER SUBSTRATESCONTAINING A BULKING AGENT, HIGH SURFACE SIZING, LOW INTERNAL SIZING ANDHAVING HIGH DIMENSIONAL STABILITY”, which is hereby incorporated byreference in its entirety; and U.S. patent application Ser. No.10/662,699, now published as U.S. Patent Publication No. 2004/0065423entitled “PAPER WITH IMPROVED STIFFNESS AND BULK AND METHOD FOR MAKINGSAME”, which is hereby incorporated by reference in its entirety.

The paper may also contain a surface sizing agent such as starch and/ormodified and/or functional equivalents thereof at a wt % of from 0.05 wt% to 20 wt % and preferably from 5 to 15 wt % based on the total weightof the substrate. Examples of modified starches include, for example,oxidized, cationic, ethylated, hydroethoxylated, etc. Examples offunctional equivalents are, but not limited to, polyvinyl alcohol,polyvinylamine, alginate, carboxymethyl cellulose, etc.

The paper may be made by contacting the expandable microspheres withcellulose fibers. Still further, the contacting may occur at acceptableconcentration levels that provide the paper substrate of the presentinvention to contain any of the above-mentioned amounts of cellulose andexpandable microspheres. More specifically, the paper substrate of thepresent application may be made by adding from 0.25 to 20 lbs ofexpandable microspheres per ton of cellulose fibers. Additionally, thecontacting may occur anytime during the papermaking process including,but not limited to, the thick stock, thin stock, head box, and coater,with the preferred addition point being at the thin stock. Furtheraddition points include machine chest, stuff box, and suction of the fanpump. In addition, the paper may be made by contacting further optionalsubstances with the cellulose fibers as well. This contacting may alsooccur anytime in the papermaking process including, but not limited tothe thick stock, thin stock, head box, size press, water box, andcoater. Further addition points include machine chest, stuff box, andsuction of the fan pump. The cellulose fibers, expandable microspheres,and/or optional components may be contacted serially, consecutively,and/or simultaneously in any combination with each other. The cellulosefibers and expandable microspheres may be pre-mixed in any combinationbefore addition to or during the paper-making process.

The paper may be pressed in a press section containing one or more nips.However, any pressing means commonly known in the art of papermaking maybe utilized. The nips may be, but are not limited to, single felted,double felted, roll, and extended nip in the presses. However, any nipcommonly known in the art of papermaking may be utilized.

The paper may be dried in a drying section. Any drying means commonlyknown in the art of papermaking may be utilized. The drying section mayinclude and contain a drying can, cylinder drying, Condebelt drying, IR,or other drying means and mechanisms known in the art. The papersubstrate may be dried so as to contain any selected amount of water.Preferably, the substrate is dried to contain less than or equal to 10%water.

The paper substrate may be passed through a size press, where any sizingmeans commonly known in the art of papermaking is acceptable. The sizepress, for example, may be a puddle mode size press (e.g. inclined,vertical, horizontal) or metered size press (e.g. blade metered, rodmetered). At the size press, sizing agents such as binders may becontacted with the substrate. Optionally these same sizing agents may beadded at the wet end of the papermaking process as needed. After sizing,the paper substrate may or may not be dried again according to theabove-mentioned exemplified means and other commonly known drying meansin the art of papermaking. The paper substrate may be dried so as tocontain any selected amount of water. Preferably, the substrate is driedto contain less than or equal to 10% water.

The paper substrate may be calendered by any commonly known calendaringmeans in the art of papermaking. More specifically, one could utilize,for example, wet stack calendering, dry stack calendering, steel nipcalendaring, hot soft calendaring or extended nip calendering, etc.While not wishing to be bound by theory, it is thought that the presenceof the expandable microspheres may reduce and alleviate requirements forharsh calendaring means and environments for certain paper substrates,dependent on the intended use thereof. During calendaring, the substratemay be subjected to any nip pressure. However, preferably nip pressuresmay be from 5 to 50 psi, more preferably from 5 to 30 psi.

The paper substrate may be microfinished according to any microfinishingmeans commonly known in the art of papermaking. Microfinishing is ameans involving frictional processes to finish surfaces of the papersubstrate. The paper substrate may be microfinished with or without acalendering means applied thereto consecutively and/or simultaneously.Examples of microfinishing means can be found in United States PublishedPatent Application 2004/0123966 and references cited therein, which areall hereby, in their entireties, herein expressly incorporated byreference.

In one embodiment of the present invention, the paper substrate of thepresent invention may be a coated paper substrate. Accordingly, in thisembodiment, the paper substrate of the present invention may alsocontain at least one coating layer, including optionally two coatinglayers and/or a plurality thereof. The coating layer may be applied toat least one surface of the paper board and/or substrate, including twosurfaces. Further, the coating layer may penetrate the paper boardand/or substrate. The coating layer may contain a binder. Further thecoating layer may also optionally contain a pigment. Other optionalingredients of the coating layer are surfactants, dispersion aids, andother conventional additives for printing compositions.

The coating layer may include a plurality of layers or a single layerhaving any conventional thickness as needed and produced by standardmethods, especially printing methods. For example, the coating layer maycontain a basecoat layer and a topcoat layer. The basecoat layer may,for example, contain low density thermoplastic particles and optionallya first binder. The topcoat layer may, for example, contain at least onepigment and optionally a second binder which may or may not be adifferent binder than the first. The particles of the basecoat layer andthe at least one pigment of the topcoat layer may be dispersed in theirrespective binders.

While the coated or uncoated paper substrate may have any SheffieldSmoothness, in one or more exemplary embodiment, the coated papersubstrate according to the present invention may have a SheffieldSmoothness that is less than 50, preferably less than 30, morepreferably less than 20, and most preferably less than 15 as measured byTAPPI test method T 538 om-1.

While the coated or uncoated paper substrate may have any Parker PrintSmoothness (10 kgf/c^(m2)), in one embodiment, the coated papersubstrate according to the present invention may have a Parker PrintSmoothness (10 kgf/c^(m2)) less than or equal to 2, preferably less than1.5, more preferably less than 1.3, and most preferably from about 1.0to 0.5 as measured by TAPPI test method T 555 om-99.

The coated paper substrate according to the present invention may havean improved print mottle as measured by 2^(nd) Cyan scanner mottle.Scanner mottle is determined using the following procedure:Representative samples are selected from pigment coated paper orpaperboard printed under controlled conditions typical of commercialoffset litho production with the cyan process ink at a reflectiondensity of 1.35±0.05. A 100 percent solid cyan print reflective image isdigitally scanned and transformed through a neural network model toproduce a print mottle index number between zero (perfectly uniform inklay with no mottle) to ten (visually noticeable, objectionable andlikely rejectable because of print mottle, a random non-uniformity inthe visual reflective density or color of the printed area). Data fromthis 2^(nd) Cyan scanner mottle system can be correlated to subjectivevisual perception (using the zero-to-ten guideline) or can betransformed into equivalent mottle values as measured with a Tobiasmottle tester from Tobias Associates using the following equation:Tobias=Scanner Mottle*8.8+188

The methods of describing the procedures and details of setting up ofthe above-mentioned equation can be found in U.S. patent applicationSer. No. 10/945,306 filed Sep. 20, 2004, which is hereby incorporated byreference in its entirety.

In a preferred embodiment, the coated or uncoated paper of paperboardsubstrate of the present invention has any 2^(nd) Cyan scanner printmottle. However, the 2^(nd) Cyan scanner print mottle may be from 0 to10, preferably not more than 6, more preferably not more than 5, andmost preferably not more than 4.

The coated paper or paperboard of this invention can be prepared usingknown conventional techniques. Methods and apparatuses for forming andapplying a coating formulation to a paper substrate are well known inthe paper and paperboard art. See for example, G. A. Smook andreferences cited therein, all of which are hereby incorporated byreference in their entireties. All such known methods can be used in thepractice of this invention and will not be described in detail. Forexample, the mixture of essential pigments, polymeric or copolymericbinders and optional components can be dissolved or dispersed in anappropriate liquid medium, preferably water.

The coating formulation can be applied to the substrate by any suitabletechnique, such as cast coating, Blade coating, air knife coating, rodcoating, roll coating, gravure coating, slot-die coating, spray coating,dip coating, Meyer rod coating, reverse roll coating, extrusion coatingor the like. In addition, the coating compositions can also be appliedat the size press of a paper machine using rod metering or othermetering techniques. In the preferred embodiments of the invention, thebasecoat coating formulation is applied using blade coaters and thetopcoat coating formulation is applied using a blade coater or air knifecoater. In the most preferred embodiments the basecoat is applied usinga stiff blade coater and the topcoat is applied using a bent bladecoater or an air knife coater.

The coated or uncoated paper is dried after treatment with the coatingcomposition. Methods and apparatuses for drying paper or paperboard webstreated with a coating composition are well known in the paper andpaperboard art. See for example G. A. Smook referenced above andreferences cited therein. Any conventional drying method and apparatuscan be used, and would be identifiable by one of skill in the art.Accordingly, these methods and apparatuses will not be described hereinin any great detail. Preferably after drying, the paper web will have amoisture content equal to or less than about 10% by weight. The amountof moisture in the dried paper or paperboard web is preferably fromabout 5 to about 10% by weight.

After drying, the coated or uncoated paper may be subjected to one ormore post drying steps, such as those described in G. A. Smookreferenced above (and references cited therein). For example, the paperweb may be calendered to improve the smoothness and improve print mottleperformance, as well as other properties of the paper such as, forexample, by passing the coated paper through a nip formed by a calender.Gloss calenders (chromed steel against a rubber roll) or hot soft glosscalenders (chromed steel against a composite polymeric surface) may beused to impart gloss to the top coated paper or paperboard surface. Theamount of heat and pressure needed in these calenders depends on thespeed of the web entering the nip, the roll sizes, roll composition andhardness, specific load, the topcoat and basecoat weights, the roughnessof the under lying rough paperboard, the binder strength of thecoatings, and the roughness of the pigments present in the coating.

The substrate and coating layer are contacted with each other by anyconventional coating layer application means, including impregnationmeans. A preferred method of applying the coating layer is with anin-line coating process with one or more stations. The coating stationsmay be any of known coating means commonly known in the art ofpapermaking including, for example, brush, rod, air knife, spray,curtain, blade, transfer roll, reverse roll, and/or cast coating means,as well as any combination of the same.

The coated substrate may be dried in a drying section. Any drying meanscommonly known in the art of papermaking and/or coatings may beutilized. The drying section may include and contain IR, air impingementdryers and/or steam heated drying cans, or other drying means andmechanisms known in the coating art. In addition, the coated substratemay be finished according to any finishing means commonly known in theart of papermaking. Examples of such finishing means, including one ormore finishing stations, include gloss calendar, soft nip calendar,and/or extended nip calendar.

These above-mentioned methods of making the paper of the presentinvention may be added to any conventional papermaking processes, aswell as converting processes, including abrading, sanding, slitting,scoring, perforating, sparking, calendaring, sheet finishing,converting, coating, laminating, printing, etc. Preferred conventionalprocesses include those tailored to produce paper substrates capable asbeing utilized as coated and/or uncoated paper products, board, and/orsubstrates.

The expandable microsphere of the present invention may be utilized inany and all end uses commonly known in the art for using paper and/orpaperboard substrates. Such end uses include the production of paperand/or paperboard packaging and/or articles, including those requiringhigh and low basis weights in the respective substrates, which can rangefrom envelopes and forms to folding carton, respectively. Further, theend product may have multiple paper substrate layers, such as corrugatedstructures, where at least one layer contains the expandable microsphereof the present invention.

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples illustrated belowwhich are provided for purposes of illustration only and are notintended to be all inclusive or limiting unless otherwise specified.

EXAMPLES Example 1 Preparation of Microspheres Using SuspensionPolymerization with a Shot Growth Method

1. Preparation of the Aqueous Phase

100 g of deionized water was added to 7.4 g of sodium chloride in a 250ml beaker. The solution was stirred until the sodium chloride wasdissolved. 6.25 g of sodium dichromate (2.5% aqueous solution) was addedto the sodium chloride solution with stirring. Stirring was continuedfor 2 minutes. 5.5 g of ethanol was then added to this solution andstirred for two minutes. Next, 5.25 g of Lupamin® 5095, a polyvinylamine commercially available from BASF, was added to a 1000 ml beaker.137.5 g of deionized water was added to the polyvinyl amine and thesolution was stirred for about 2 minutes. 40.4 g of colloidal silica wasplaced into a separate 250 ml beaker, 130 g of deionized water was addedthereto, and the resulting solution was stirred for about 2 minutes. Thecolloidal silica solution was then slowly added to the 1000 ml beakercontaining the polyvinyl amine solution and stirring was continued for 5minutes. After 5 minutes, the sodium chloride/sodium dichromate/ethanolsolution was added to the 1000 ml beaker with stirring. Stirring wascontinued for an additional 5 minutes. Concentrated hydrochloric acidwas added to bring the pH of the aqueous phase to 3.5, and the solutionwas stirred for an additional 5 minutes.

2. Preparation of the Organic Phase

195.18 g of vinylidene chloride (VDC) was added to a flask that waschilled to −30° C. 113.9 g of isobutane was added to the chilled flask.The resulting mixture was poured into a large pressure addition burettethat had been purged with argon and chilled in a freezer. The funnelused to pour the mixture into the burette was chilled with liquidnitrogen to minimize evaporation of the isobutane. Next, 35.58 g ofacrylonitrile (AN), 2.6 g of 2,2′-azobisisobutyronitrile (AIBN), 1.78 gof trallyl cyanurate (TAC), 7.12 g of methyl methacryhlate (MMA) wasadded to a flask at room temperature. The flask was shaken to dissolvethe AIBN and the TAC. The mixture was then poured into a small pressureaddition burette that had been purged with argon. The mixture in thesmall pressure addition burette was transferred to the big pressureburette by pressurized argon. The organic phase present in the bigburette was mixed by turning the burette up and down 10 times.

3. Disperse the Organic Phase into the Aqueous Phase

The aqueous phase formed in step 1 above was added to a 3-liter reactorthat had been purged with argon. The aqueous phase was stirred withinthe reactor at a 1,500 rpm shear rate while the organic phase was slowlyadded by pressurized argon (over a period of 10 minutes). After theaddition of the organic phase was completed, the mixture was agitatedfor 15 minutes. A stable dispersion was formed. The shear rate was thenreduced to 90 rpm.

4. Polymerization Reaction

The polymerization reaction was initiated by heating the mixture in thereactor to 60° C. The reaction temperature was permitted to remain at60° C. for 12 hours. The pressure inside the reactor during thepolymerization was maintained from bout 5 to about 8.5 bars. After 12hours of polymerization, a solution containing 1.25 g of AIBN dissolvedin 8.5 g of AN was added to the reactor by using a small (250 ml)pressure addition burette and pressurized argon. The temperature of themixture was increased to 70° C. The mixture was them permitted to reactfor 8 hours. The pressure inside the reactor during this 8 hour periodwas about 8.5 to about 10.5 bars.

5. Post Polymerization Treatment

After the 8 hour polymerization period, the reactor was cooled to roomtemperature and the pressure was released. The mixture within thereactor was transferred to a 2,000 ml beaker. The mixture was thenfiltered using a 150 μm meter mesh screen to remove any coarse coagulum.Next, the microspheres were filtered using a regular vacuum filtrationassembly with a funnel, grade 4 qualitative paper, and a filtrationflask connected to a hose vacuum. The microspheres were washed threetimes with deionized water. The ease in the filtration flask was pouredout and the microspheres in the funnel were permitted to dry overnight.The dried microspheres were then weighed and the yield was calculated(about 86% based on the weight of the microspheres and the organicphase).

Similar processes were used for the other samples as set forth inTable 1. The same amount of AIBN and AN for the “shot growth” was usedfor all the samples in Table 1. The same reaction time both before andafter the shot of AN was used for all the samples set forth in Table 2with the exception of Sample 10, which had 8 hours of polymerizationbefore the shot of AN and 8 hours of polymerization after the shot andSample 13, which had 6 hours of polymerization before the shot of AN and7 hours of polymerization after the shot.

TABLE 2 Organic Phase Components VDC AN TAC i-butane AIBN MMA Agitator(% by (% by (% by (% by (% by (% by Speed Sample weight) weight) weight)weight) weight) weight) (RPM) 1 54.8 10 0.5 32 0.73 2.0 1500 2 53.8 101.5 32 0.73 2.0 1500 3 47.8 16 1.5 32 0.73 2.0 1500 4 51.3 13 1.0 320.73 2.0 1500 5 48.8 16 0.5 32 0.73 2.0 1500 6 55.1 10 0.2 32 0.73 2.01500 7 57.42 7.5 0.35 32 0.73 2.0 1500 8 59.8 5 0.5 32 0.73 2.0 1500 954.8 10 0.5 32 0.73 2.0 1500 10 54.8 10 0.5 32 0.73 2.0 1500 11 47.8 161.5 32 0.73 2.0 1200 12 48.8 16 0.5 32 0.73 2.0 1200 13 54.8 10 0.5 320.73 2.0 1500 14 51.3 13 1.0 32 0.73 2.0 1200

TABLE 3 Aqueous Phase Components Sodium Polyvinyl Colloidal SodiumChloride Amine Silica Dichromate Ethanol Hydrochloric Deionized (% by (%by (% by (% by (% by Acid Water weight) weight) weight) weight) weight)(pH 3.5) 85.0 1.7 1.2 9.3 1.4 1.3 —

The particle size distributions of these microspheres were characterizedby the Malvern Mastersizer particle size analyzer. Table 4 sets forththe results of these particle size analyses.

TABLE 4 Particle Size of Microspheres Sample D (v, 0.1) D (v, 0.5) D (v,0.9) 1 5.02 20.49 36.07 2 9.91 20.22 38.5 3 7.51 18.08 37.69 4 6.1320.84 40.96 5 6.37 19.61 44.52 6 12.78 23.96 39.63 7 7.81 21.71 41.04 87.13 20.76 38.99 9 5.08 17.95 33.73 10 4.72 19.09 24.21 11 9.73 27.1453.78 12 7.45 24.17 42.66 13 9.50 20.64 34.54 14 7.2 21.59 44.37

Example 2 Microspheres in Papermaking Process

Paper samples containing the expandable microspheres described above inExample 1 were produced. Dry samples of the microspheres from Example 1were formed into a 2% slurry by adding the dry microspheres tovigorously stirring water with a cowles mixer. After 10 minutes ofmixing, the slurry was ready for a paper making machine.

The paper machine was started and uncoated freesheet paper grade with atargeted basis weight of 80 g/m² was produced. The pulp blend was 65/35bleached hardwood kraft pulp/bleached softwood kraft pulp. The papercontained 10% precipitated calcium carbonate filler and 15 lb/T cationicstarch. An ethylated starch was used at the size press with a targetedpick-up of 80 lb/T.

Microspheres from Example 1 were fed into the paper machine at atargeted dose of 15 lb/T. The addition point was before the 15 lb/T ofcationic starch. 20 minutes after adding the first microsphere sample,the paper machine had come to equilibrium and samples of paper at thereel were collected. All 14 samples of microspheres described in Example1 were run on the paper machine in a similar manner. The paper sampleswere then calendered on a sheet fed off-machine calender to achieve thedesired smoothness. The basis weight, smoothness, and stiffness weremeasured on the calendered paper. In this manner, bulk and stiffnesswere compared between the different microsphere-containing papers. Theincrease in bulk compared to the paper made without any microspheres wascalculated and recorded. The results are set forth in Tables 5 and 6.

TABLE 5 Uncalendered Paper Results Uncalendered Basis Weight % BulkSample (g/m²) Bulk Increase Control 81.4 2.13 0.0 1 83.1 2.33 9.1 2 82.52.31 8.1 3 82.0 2.25 5.6 4 80.7 2.40 12.5 5 80.7 2.51 17.8 6 80.2 2.328.6 7 80.8 2.21 3.6 8 80.8 2.20 2.9 9 82.4 2.44 14.5 10 82.6 2.47 15.611 84.5 2.29 7.2 12 80.7 2.45 16.4 13 79.2 2.50 17.2 14 79.2 2.36 10.6

TABLE 6 Calendered Paper Results Calendered to 250 Sheffield GM BasisWeight % Bulk Stiffness Sample (g/m²) Bulk Increase Index CaliperControl 81.4 1.69 0.0 133.1 5.4 1 83.1 1.78 5.7 131.0 5.9 2 82.5 1.817.2 130.2 5.9 3 82.0 1.84 9.1 142.2 5.9 4 80.7 1.86 10.0 134.7 5.8 580.7 1.91 13.4 138.9 6.1 6 80.2 1.86 10.1 140.8 6.0 7 80.8 1.86 10.2147.7 5.8 8 80.8 1.82 7.6 144.8 5.8 9 82.4 1.89 12.1 134.9 6.1 10 82.61.87 10.8 133.1 6.2 11 84.5 1.81 7.1 129.1 6.0 12 80.7 1.88 11.4 137.26.0 13 79.2 1.91 13.5 135.8 6.0 14 79.2 1.87 11.0 136.3 5.8

The results from Example 2 showed that all of the microspheres producedin Example 1 demonstrated in increase in uncalendered and calenderedbulk at 250 Sheffield smoothness.

The invention of this application has been described above bothgenerically and with regard to specific embodiments. Although theinvention has been set forth in what is believed to be the preferredembodiments, a wide variety of alternatives known to those of skill inthe art can be selected within the generic disclosure. The invention isnot otherwise limited, except for the recitation of the claims set forthbelow.

1. An expandable microsphere comprising: a gas impermeable shellencapsulating at least one blowing agent, said gas impermeable shellincluding: a first polymeric layer surrounding said at least one blowingagent; and a second polymeric layer at least substantially encapsulatingsaid first polymeric layer, wherein said second polymeric layer includesa homo- and/or co-polymer of at least one monomer selected from thegroup consisting of a monomer having a weight average Tg of at least 85°C. and a chemically reactive monomer; wherein said at least one monomeris selected from the group consisting of acrylonitrile, vinylidenechloride, methyl methacrylate, tetraethylene glycol dimethacrylate,2,3-epoxypropyl acrylate, methacrylonitrile, glycidyl methacrylate,methacrylic acid, and vinyl benzyl chloride; and wherein said secondpolymeric layer has a weight average Tg that is greater than a weightaverage Tg of the first polymeric layer.
 2. The expandable microsphereof claim 1, wherein said first polymeric layer comprises polymers formedfrom monomers selected from nitrile containing monomers, acrylic estermonomers, methacrylic ester monomers, vinyl esters, vinyl halidemonomers and combinations thereof.
 3. The expandable microsphere ofclaim 2, wherein said first polymeric layer comprises at least onenitrile containing monomer, at least one methacrylic ester monomer, andat least one vinylidene halide monomer.
 4. The expandable microsphere ofclaim 2, wherein said monomers have a weight average Tg less than 85° C.5. The expandable microsphere of claim 2, wherein said first polymericlayer further comprises a crosslinking monomer.
 6. The expandablemicrosphere of claim 2, further comprising a cationic species covalentlybonded to said second polymeric layer.
 7. The expandable microsphere ofclaim 4, wherein said second polymeric layer has a larger amount of saidmonomer having a weight average Tg of at least 85° C. compared to anamount of said monomers having a weight average Tg of at least 85° C.present in said first polymeric layer.
 8. The expandable microsphere ofclaim 1, wherein said first and second polymeric layers are covalentlyattached.
 9. The expandable microsphere of claim 1, wherein saidexpandable microsphere has an expanded volume average diameter of lessthan 50 microns.
 10. The expandable microsphere of claim 1, wherein saidexpandable microsphere has a temperature of onset expansion from about60° C. about 105° C.
 11. The expandable microsphere of claim 1, whereinsaid expandable microsphere has a temperature of shrinkage greater thanabout 105° C.
 12. The expandable microsphere of claim 1, having anexpanded volume average diameter of 1 μm to 100 μm.
 13. A method offorming an expandable microsphere comprising: mixing primary monomersselected from nitrile containing monomers, acrylic ester monomers,methacrylic ester monomers, vinyl esters, vinyl halide monomers andcombinations thereof, at least one blowing agent, a crosslinkingmonomer, a polymerization initiator, and a stabilizer system in areaction vessel for a period of time sufficient to achieve anapproximate 90% polymerization of said primary monomers and form a firstpolymeric layer surrounding said blowing agent; and adding a secondarymonomer selected from monomers having a weight average Tg of at least85° C. and chemically reactive monomers to said reaction vessel to forma second polymeric layer at least substantially surrounding said firstpolymeric layer and form an expandable microsphere; to form anexpandable microsphere comprising: a gas impermeable shell encapsulatingat least one blowing agent, said gas impermeable shell including: afirst polymeric layer surrounding said at least one blowing agent; and asecond polymeric layer at least substantially encapsulating said firstpolymeric layer, wherein said second polymeric layer includes a homo-and/or co-polymer of at least one monomer selected from the groupconsisting of a monomer having a Tg of at least 85° C. and a chemicallyreactive monomer; wherein said at least one monomer is selected from thegroup consisting of acrylonitrile, vinylidene chloride, methylmethacrylate, tetraethylene glycol dimethacrylate, 2,3-epoxypropylacrylate, methacrylonitrile, glycidyl methacrylate, methacrylic acid,and vinyl benzyl chloride; and wherein said second polymeric layer has aweight average Tq that is greater than a weight average Tg of the firstpolymeric layer.
 14. The method of claim 13, wherein said reactionvessel further includes at least one member selected from a salt, aphase partitioner, an inhibitor an acid and water.
 15. The method ofclaim 13, further comprising: purging said reaction vessel to removeoxygen.
 16. The method of claim 13, further comprising: adding acationic species to said microsphere such that said cationic speciescovalently bonds to said second polymeric layer.
 17. The method of claim13, wherein the microsphere has an expanded volume average diameter of 1μm to 100 μm.
 18. A composition comprising the expandable microsphere ofclaim 1 and a plurality of cellulose fibers.
 19. A paper comprising: aweb of cellulose fibers; and a plurality of expandable microspheres,said expandable microspheres including: a gas impermeable shellencapsulating at least one blowing agent, said gas impermeable shellincluding: a first polymeric layer surrounding said at least one blowingagent; and a second polymeric layer at least substantially encapsulatingsaid first polymeric layer, wherein said second polymeric layer includesa homo- and/or co-polymer of at least one monomer selected from thegroup consisting of a monomer having a weight average Tg of at least 85°C. and a chemically reactive monomer; wherein said at least one monomeris selected from the group consisting of acrylonitrile, vinylidenechloride, methyl methacrylate, tetraethylene glycol dimethacrylate,2,3-epoxypropyl acrylate, methacrylonitrile, glycidyl methacrylate,methacrylic acid, and vinyl benzyl chloride; and wherein said secondpolymeric layer has a weight average Tg that is greater than a weightaverage To of the first polymeric layer.
 20. The paper of claim 19,wherein the microsphere has an expanded volume average diameter of 1 μmto 100 μm.
 21. A method of making a paper substrate, comprisingcontacting a plurality of cellulose fibers with at least one expandablemicrosphere according to claim 1 prior to or at a head box, at the sizepress, or at a coater.